drm/bridge: analogix_dp: properly handle zero sized AUX transactions

Address only transactions without any data are valid and should not
be flagged as short transactions. Simply return the message size when
no transaction errors occured.

CC: stable@vger.kernel.org
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240318203925.2837689-1-l.stach@pengutronix.de